1. Field of the Invention
This invention relates to FM radio transmission systems and methods, and more particularly to FM systems using special techniques to reduce noise and accurately match the received radio signal with the original audio signal submitted for transmission.
2. Description of the Prior Art
Commercial frequency modulated (FM) signals are broadcast in the radio spectrum from 88 to 108 MHz. There are 100 channels, each 200 KHz wide. The original audio signal is encoded as a frequency modulated signal with a maximum allowed frequency deviation of .+-.75 KHz. In stereo broadcasting the signal is encoded so that non-stereo receivers will receive a monaural signal equal to the sum of the left (L) and right (R) signals. This is accomplished by using the L-R signal in the form of a double sideband suppressed carrier signal to modulate a 38 KHz subcarrier, and adding the resulting signal to the L+R signal along with a 19 KHz pilot tone. The channel's main carrier is then FM modulated with the summed signal. The audio bandwidth is limited in all cases to a band of frequencies from about 50 Hz to 15 KHz. The 38 KHz subcarrier for the L-R signal is suppressed to minimize the required FM deviation of the main carrier during broadcast, while a 19 KHz pilot tone is phase-locked to the 38 KHz subcarrier and broadcast along with the composite signal to allow synchronous demodulation of the L-R signal.
A conventional transmission system is illustrated in FIG. 1. The left and right stereo audio signals are applied to a matrixing circuit 2 which outputs the sum L+R and difference L-R of the two signals. The L-R signal goes to one input of a four quadrant multiplier 4, the other input to which is taken from a 38 KHz oscillator 6. Multiplier 4 produces an output in the form of a double sideband suppressed carrier modulation of the L-R signal on the 38 KHz reference signal. The 38 KHz signal is also frequency divided by two in a frequency divider circuit 8 to generate a 19 KHz pilot signal. A summing circuit 10 adds the 19 KHz pilot signal, the 38 KHz signal modulated by the L-R signal, the L+R signal, and a conventional subsidiary communication authorization (SCA) 60-75 KHz signal. The resulting composite signal is delivered to an FM modulator 12, where it modulates the carrier signal at the center of the broadcast band.
The frequency spectrum of the composite broadcast signal is illustrated in FIG. 2. Taking the carrier frequency as a zero reference, the spectrum to 15 KHz is occupied by the L+R signal, with a narrow subaudio deadband extending from the carrier up to the beginning of the audio level, typically about 50 Hz. The 19 KHz pilot tone is transmitted above the audio L+R level, but below the double sideband L-R level. The L-R double sidebands are centered on 38 KHz as illustrated, and extend .+-.15 KHz from this frequency, or from about 23 to 53 KHz. A relatively narrow subaudio deadband of about .+-.50 Hz is centered on the 38 KHz subcarrier between the two L-R sidebands; the width of this deadband is somewhat exaggerated in FIG. 2 for purposes of illustration. Beyond the L-R sideband the SCA signal is transmitted in a frequency band of 60-75 KHz.
FIG. 3 is a simplified diagram of a conventional receiver used to demodulate the transmitted composite stereo signal. The received signal is fed to a 76 KHz oscillator 14 which phase-locks to the 19 KHz pilot tone. The output of oscillator 14 is frequency divided by flip-flop circuit 36 to produce a 38 KHz square wave in-phase with the suppressed 38 KHz carrier. The 38 KHz signal flip-flop 16 and the received composite signal are both delivered to a synchronous demodulator circuit 18, which acts as a synchronous signal directing switch. Demodulation is accomplished by switching the input to the synchronous demodulator to a left channel output during the positive portion of the 38 KHz reference signal, and to a right channel output during the negative portion of the 38 KHz reference signal. This technique is illustrated in FIG. 4, in which the synchronous demodulator is shown as being implemented by a switch 20 which is exercised at a 38 KHz rate. Since the switching occurs above audio frequency, its effects are easily removed from the output signals by providing low pass filters 22 and 24 in the left and right channel output lines to pass the audio signals but filter out the higher frequency switching signals. FM demodulation can be accomplished commercially by single chip integrated circuits which receive the composite signal and produce left and right channel output audio signals with this technique. Typical chips may be used for this purpose are the National Semiconductor Corporation LM1800, and the RCA Corporation CA3090AQ.
The useful dynamic range of commercial FM broadcast is typically 60 to 70 dB. However, digital audio recordings typically have dynamic ranges of greater than 90 dB. In order to fit a 90 dB audio signal range into a 60-70 dB transmission range, and also to increase the "loudness" of a broadcast for a given allowable transmission power, broadcasters typically "compress" and "limit" their signals to place the average signal as far above the noise floor as practical. Unfortunately, this type of signal encoding has several undersirable effects. Since the signal levels are boosted up towards a maximum permissible level during heavy limiting, the difference between transmitted signal levels tends to be much less than the difference between the original signal levels. This compression of the diferences in signal amplitudes at the transmission end makes it much more difficult to differentiate between different signal levels to restore the original audio signal at the receiving end. Conventional companding techniques are incapable of accurately determining the proper expansion at the receiver because the changes in transmitted signal amplitude are so small during heavy limiting. In addition, the use of limiters and compressors to boost the transmitted signal level in general so as to achieve a louder sound results in clipping signal peaks to avoid overshoot beyond the maximum transmission level permitted by governmental authorities. The advent of digital audio discs with their wider dynamic range makes it particularly desirable to increase the effective dynamic range of a commercial FM broadcast.